1. Field of the Invention
This invention relates to a system for the reduction of harmful exhaust emissions from diesel engines, and more particularly to a system for increasing the effectiveness of the oxidation of the oxidizable components in the exhaust emissions.
2. Description of Related Art
Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide ("CO"), unburned hydrocarbons ("HC") and nitrogen oxides ("NO.sub.x "), but also condensed phase materials (liquids and solids) which constitute the so-called particulates or particulate matter ("PM"). The total particulate matter ("TPM") emissions are comprised of three main components. One component is the solid, dry, solid carbonaceous fraction or soot. This dry carbonaceous matter contributes to the visible soot emissions commonly associated with diesel exhaust. A second component of the TPM is the soluble organic fraction ("SOF"). The soluble organic fraction is sometimes referred to as the volatile organic fraction ("VOF"), which terminology will be used herein. The VOF may exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust, and are generally present as condensed liquids at the standard particulate collection temperature of 52.degree. C. in diluted exhaust, as prescribed by a standard measurement test, such as the U.S. Heavy Duty Transient Federal Test Procedure, discussed further below. These liquids arise from two sources: (1) lubricating oil swept from the cylinder walls of the engine each time the pistons go up and down; and (2) unburned or partially burned diesel fuel.
The third component of the particulates is the so-called sulfate fraction. Diesel fuel contains sulfur, and even the low sulfur fuel available in the U.S. may contain 0.05% sulfur. Upon combustion of the fuel in the engine, nearly all of the sulfur is oxidized to sulfur dioxide which exits with the exhaust in the gas phase. However, a small portion of the sulfur, perhaps 2-5%, is oxidized further to SO.sub.3, which in turn combines rapidly with water in the exhaust to form sulfuric acid which collects as a condensed phase with the particulates as an aerosol, or is adsorbed onto the other particulate components, and thereby adds to the mass of TPM.
Emissions from diesel engines have been under increasing scrutiny in recent years and standards, especially for particulate emissions, have become stricter. In 1994 the particulate emission standards in the U.S. for new engines allowed no more than a total of 0.1 grams per brake horse power hour (g/BHP-h). For diesel engines in buses operating in congested urban areas the particulate emissions standard was even stricter, 0.07 g/BHP-h TPM. Both of these standards were seen as significant reductions relative to the prior particulate emission standard of 0.25 g/BHP-h which had been in effect since 1991. Starting in 1994, for the first time, engine technology developments alone were found to be incapable of meeting the new standards, and for some engines aftertreatment technology, for example, diesel oxidation catalyst (DOC) units, as discussed further below, were necessary.
Current engines are generally capable of meeting the 1994 NO.sub.x emissions standards of 5.0 g/BHP-h, but by only a slim margin. Diesel engines, because they operate with a great excess of combustion air (lean exhaust) typically have emissions of CO and gas phase HC's which are well below the 1994 emissions standards of 15.5 g/BHP-h and 1.3 g/BHP-h, respectively. Therefore, the key emission control concerns for diesel engines now and for the immediate future are the reduction in particulates (TPM) and NO.sub.x emissions.
Emissions of NO.sub.x from diesel engines can be reduced by retarding injection timing. However, this is accompanied by a corresponding increase in particulate emissions, particularly of the dry carbon or soot portion. Emissions of NO.sub.x can also be reduced by applying exhaust gas recirculation (EGR) technology. However, this is also accompanied by a corresponding increase in particulate emissions. Thus, both of these engine technologies are constrained by a trade-off or balance between TPM and NO.sub.x emissions.
Additional EPA requirements went into effect at the beginning of 1995 which apply to urban buses equipped with engines manufactured prior to 1994. These requirements apply to engines in service when they come due for rebuilding. Following engine rebuild the requirements must be met. One portion of these requirements specifies that if technology can be demonstrated for particulate reduction for these pre-1994 bus engines, that technology would be mandated for use on those engines for which it is certified. Two tiers of such technology/emissions reduction attainment were promulgated including:
1. Meet the 1994 Emissions Standard of 0.1 g/BHP-h TPM, with a technology cost cap of about $8,000. PA1 2. Reduce Engine-Out TPM Emissions by at least 25%, with a technology cost cap of about $3,000.
The first of the above attainment levels, which is considered the stricter of the two requirements, if demonstrated and certified, takes precedence. Thus, the 25% TPM reduction tier was considered a "fall-back" position, if the 0.1 g/BHP-h TPM tier could not be met. It is clear from the strict emissions requirements for new diesel engines used in urban buses and the attainment requirement for pre-1994 bus engines that a major challenge exists for this type of application.
Diesel engines used in urban bus applications in the U.S. are of many types, both two-cycle and four-cycle, supplied by a range of engine original equipment manufacturers (OEM's). However, a large percentage of urban transit buses have two-cycle engines from one manufacturer (Detroit Diesel Corp.). The emissions reduction system of this invention is considered to be applicable to any diesel engine for lowering emissions and the level of emissions reduction attained is expected to be dependent on the specific engine, its operating parameters and baseline engine-out emissions. However, this invention has been found to be especially useful for two-stroke diesel engines, and as demonstrated herein, can be used with such engines manufactured prior to 1994 to bring them into compliance with the 1994 particulate emissions standard of 0.1 g/BHP-h TPM, as discussed above.
Oxidation catalysts comprising a platinum group metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both HC and CO gaseous pollutants and particulates, i.e., soot particles, by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts have generally been contained in units called diesel oxidation catalysts (DOC's), or more simply catalytic converters or catalyzers, which are placed in the exhaust train of diesel power systems to treat the exhaust before it vents to the atmosphere. However, by the time the exhaust gas reaches the catalyzer, it has generally lost a considerable amount of heat, both by radiation through the engine and exhaust system walls, and by intentional power transfer at the turbocharger. Because the efficiency of such catalytic oxidation processes is generally a direct function of the gas temperature, such temperature losses can have a significant negative impact on the effectiveness of the catalyzer.
One approach to improving the effectiveness of the catalyzer is to maintain the exhaust temperature at as high a level as possible, from the combustion chamber and through the connecting exhaust train to the catalyzer. Heat-insulating structures and heat-insulating coatings, i.e., thermal barrier coatings have been employed by those skilled in the art to enhance the thermal efficiency of internal combustion engines by permitting more complete fuel burning at higher temperatures. Typically, such heat-insulating coatings have been applied to all of the chamber surfaces, including the cylinder walls and head and piston combustion faces to prevent heat loss. Heat-insulating structures and heat-insulating coatings have also been used in automobile exhaust systems to maintain high exhaust temperatures required by thermal reactors and catalytic converters, thus reducing the emission of unburned hydrocarbons emitted into the atmosphere as an undesirable component of exhaust gas.
U.S. Pat. No. 5,384,200 is directed to particular thermal barrier coatings and methods of depositing such coatings on the surfaces of combustion chamber components. As discussed in that patent, insulating the combustion chamber components reduces the amount of heat loss in the engine. The higher temperature in the combustion chamber results in a more complete combustion of the fuel in the chamber, and also results in a hotter exhaust being delivered to any downstream catalytic converters to promote more effective oxidation of the oxidizable components of the exhaust stream.
The use of thermal barrier coatings has also been suggested for engine components other than in-cylinder surfaces. In a paper entitled "High Performance Coatings for Diesels and Other Heat Engines", by Roy Kamo, presented at the Thermal Spray Coatings Conference, Gorham Advanced Materials Institute, Orlando, Fla., on Sept. 12-14, 1993, it is suggested that engine performance can be improved by applying thermal barrier coatings to various engine components. In addition to in-cylinder surfaces such as the piston crown and cylinder head, the article also suggests the exhaust port, exhaust manifold and turbocharger housing.